The present invention relates to a fixing device for fixing toner images in electrophotographic copying machines or other equipment and an imaging system in which such a fixing device is built and, more particularly, the present invention relates both to a heated roller type fixing device designed to be coated thereon with a releasant for preventing "offsetting" of toner onto a fixing or pressure roller and to an imaging system with such a fixing device built in it.
It has so far been known in the art that a part of the releasant (oil) fed to the fixing or pressure roller is transferred onto copying paper when the paper is fixed with a heated roller type fixing device. The oil transferred onto the copying paper diffuses for a relatively short period of time into the fibers of the copying paper through and amongst the toner particles or through pinholes. The oil which is transferred onto a toner image-free surface portion of the paper, i.e., the oil transferred directly onto the surface of the paper, also diffuses into the paper's fibers in a similar manner as mentioned above, but the releasant produces no appreciable influence on the resulting image.
However, a fixing device--used with a color copying machine, esp., one designed to reconstruct a multicolor image by superposition of three- or four-color toners--is significantly different from that used with a conventional, black & white type copying machine for the following reasons:
(1) the toners must be fixed in the form of a thick layer so as to reconstruct a multicolor image by superposition; and
(2) the toners, differing in color, must be heated well and fluidized so as to develop colors by fixing and color mixing.
These are likely to present conditions under which the fixing device brings about such an offsetting phenomenon as already referred to.
In order to avoid such offsetting, it is required for a color copying machine to have a fixing device with a high releasability. To this end, use may be made of a fixing device including a fixing roll which is coated thereon with a rubbery elastomer such as silicone rubber and to which a large amount of a releasant, i.e., silicone oil is fed in the form of a release layer.
Feeding a large amount of silicone oil to the fixing roll, however, results in the presence of a considerable amount of silicone oil on the surface of the sheet which also has a thick toner layer thereon. This silicone oil is, in turn, left on the toner surface over a relatively long period of time without diffusing into the paper fibers, because the toner particles have been well fused into coalescence. The oil cannot pass through the coalesced toner into the paper. When fingers come in contact or touch with such an image, there will be left fingerprints or other stains. The amount of oil residue changes gradually with the passage of time; this means that both the density and gloss of the image decrease with time, making the image unstable.
When the toner layer cracks by external force, i.e., by the expansion and shrinkage or deformation of the paper by moisture absorption, the oil diffuses relatively quickly into the paper fibers through the cracks, so that the toner layer separates into two regions, one having the oil thereon and the other not. The region having the oil thereon has a high image density, but the region having no oil thereon has a low image density. Thus, even though the image is initially of uniform density, it will suffer density variations due to an uneven distribution of oil and so will become unpleasant to look at.
In view of the foregoing, an object of this invention is to provide a fixing device capable both of eliminating image density variations which may otherwise be caused by an uneven distribution of a releasant remaining on the surface of the toner on the copying paper and of assuring an image density which does not change with the lapse of time and so is stable. It is another object to provide an imaging system in which such a fixing device is incorporated.